The present invention relates to nitridization of a metal or metal silicide layer to form a metal nitride or metal silicon nitride barrier layer in a contact hole or a via of an integrated circuit (IC). More particularly, the present invention relates to nitridization of a metal or metal silicide layer in a high density, low pressure plasma system to form a metal nitride or metal silicon nitride barrier layer having a substantially uniform composition along a depth of a contact hole or a via of an integrated circuit (IC).
In the semiconductor fabrication art, (e.g., the fabrication of integrated circuits or flat panel displays from substrates) refractory metal nitrides, e.g., titanium nitride (TiN), and refractory metal silicon nitrides, e.g., tungsten silicon nitride (W--Si--N), are commonly employed as barrier layer materials inside a contact hole or a via. Contact holes and vias are openings that are typically formed in a dielectric layer, such as a silicon dioxide layer. By way of example, FIG. 1A shows a partially fabricated integrated circuit 10 including a contact hole 16, which is formed by etching through a silicon dioxide layer 14 (hereinafter referred to as "oxide layer 14") to provide an opening to an underlying substrate layer 12. In subsequent IC fabrication steps, contact hole 16 may be filled with conductive materials such as tungsten, copper or aluminum, to ultimately form contact plugs that provide a conductive pathway between an IC substrate and, for example, a polysilicon layer disposed above the IC substrate.
Vias are similarly formed and filled to provide conductive pathways between successive layers of metallization disposed above the IC substrate.
Before the contact hole or via is filled with conductive material, a barrier layer may be fabricated to prevent the diffusion of conductive material such as aluminum from the conductive into the silicon substrate layer. By way of example, a barrier layer conformally deposited on the surface of IC 10, partially fills contact hole 16 and effectively prevents the diffusion of particles from subsequently deposited aluminum metal into silicon substrate layer 12. It is well known in the art that the ingress of such conductive particles into the silicon substrate layer can increase the conductivity of the silicon substrate layer and lead to catastrophic device failures.
Traditionally, such barrier layers have been fabricated on IC surfaces by the well known technique of chemical vapor deposition (CVD). Briefly, in chemical vapor deposition (CVD), a chemical containing atoms of the material to be deposited reacts with another chemical to produce the desired material on the substrate surface while the byproducts of the reaction are removed from the reaction chamber.
Fabrication of a titanium nitride (TiN) barrier layer, for example, may begin when partially fabricated IC 10 of FIG. 1A is secured on a chuck in a deposition chamber. Next, an inert atmosphere is created in the deposition chamber, where the substrate may be maintained at a high enough temperature, e.g., about 400.degree. C., to provide the necessary energy for the reactant gases to react and deposit on the substrate surface. Next, the reactant gases, which include ammonia (NH.sub.3), hydrogen (H.sub.2) and organometallic gases containing titanium (Ti) gas may be introduced into the chamber. Such conditions are maintained inside the CVD chamber for so long as it is required to deposit a TiN layer of appropriate thickness.
A CVD process for fabricating a barrier layer, such as a TiN layer, suffers from several drawbacks, however. By way of example, CVD of a barrier layer subjects the IC substrate to high temperatures, which require a high thermal budget and are often incompatible with low temperature metal alloys, such as aluminum alloys, inside an IC. As a further example, the use of organometallic gases make it likely that there may be carbon inclusion in the TiN barrier layer, thereby undesirably lowering the conductivity of the barrier layer and contact plug. Fabrication of a barrier layer by CVD, therefore, also runs the risk of rendering the IC inoperable.
To remedy these problems, the barrier layer is currently fabricated by reactive sputtering. FIG. 1B shows a reactor system 100 typically employed for carrying out reactive sputtering to fabricate a barrier layer. Reactor 100 includes a chamber 108, in which a partially fabricated IC 10, also shown in FIG. 1A, is disposed above a chuck 102. Inside chamber 108, an electrically biased metal target 106 is mounted above chuck 102. The composition of metal target 106 usually depends on the kind of barrier layer that is to be formed. If a TiN barrier layer is to be fabricated, for example, then metal target 106 may include titanium (Ti). Chamber 108 may also come fitted with a gas inlet 110 and outlet (not shown to simplify illustration). Gas inlet 110 is designed to supply reactive gases inside chamber 108 and gas outlet may be designed to evacuate gaseous byproducts from chamber 108.
A typical reactive sputtering process in reactor 100 of FIG. 1B begins when partially fabricated IC 10 is secured on chuck 102. Vacuum conditions are then created in chamber 108, before a reactive gaseous mixture including argon (Ar) and nitrogen (N.sub.2) is introduced into chamber 108 via gas inlet 110. Next, the reactive gas is ionized by a radio frequency generator, for example, producing positively charged nitrogen and argon ions. The argon ions are attracted and accelerate towards target 106, which is bombarded with the radio frequency-excited argon ions. Consequently, some atoms and molecules are "knocked off" target layer 106 and the dislodged target material, e.g. titanium ions, reacts with the nitrogen ions in the gas phase to produce metal nitride (e.g. TiN), which deposits on substrate 104 and forms the barrier layer.
Unfortunately, the reactive sputtering process described above also has several drawbacks. By way of example, it is difficult to sputter deposit a barrier layer of uniform composition throughout the depth of the contact hole or via, as the IC technology moves to smaller critical dimensions, e.g. on the order of 0.35 .mu.m to 0.13 .mu.m or smaller, and greater feature depth, e.g. approaching 3 .mu.m in some instances. Contact holes and vias realized in this technology have aspect ratios as high as about 4:1 and about 5:1. For such high aspect ratios, the collision frequency between the nitrogen and titanium ions varies along the depth of the contact hole or via. By way of example, the collision frequency between the nitrogen and titanium ions near the top of contact hole 16 of FIG. 1A will be different than that near the bottom or some distance below the top of the contact hole. As a result, the reaction rate of forming the barrier layer will vary along the depth of contact hole 16 of FIG. 1A. The resulting barrier layer has a nonuniform composition, i.e. nitrogen and titanium ions are present in the barrier layer composition in different stoichiometric ratios, throughout the depth of the contact hole. Barrier layers with nonuniform composition throughout the depth of the contact hole are undesired for many reasons. By way of example, those skilled in the art will recognize that the performance of a barrier layer with nonuniform composition will be unpredictable. It is likely that the barrier layer composition over some areas of the contact hole will not effectively prevent diffusion of conductive particles, such as aluminum metal particles, into the substrate layer, thereby making the IC more susceptible to device failure.
As another example, the reactive sputtering process described above suffers from poor repeatability, i.e. the composition of the barrier layer fabricated in the contact holes or vias of few initial ICs will be different from that fabricated in the contact holes or vias of subsequent ICs. The barrier layer composition changes because the metal target layer in the reactor chamber undergoes a compositional change due to nitridization of the target layer after the fabrication of barrier layers in the few initial ICs. In other words, during the constant bombardment of the metal target layer by argon (Ar) ions, while fabricating barrier layers in the few initial ICs, some of the nitrogen ions react with the metal target layer, thereby causing the nitridization of the target layer. Thereafter, barrier layers formed in subsequent ICs by the contaminated metal target layer may have a different composition. As mentioned above, inadequate barrier layer composition (in subsequent ICs) may lead to device failures.
As yet another example, reactive sputtering also runs the risk of undesired excessive gas phase nucleation, during which large particles of the barrier layer, e.g. TiN, are formed in the gas phase before such particles strike the substrate surface. Of course one skilled in the art would appreciate that appropriately sized particles should strike the substrate surface for barrier layer deposition.
Therefore, what is needed is an improved nitridization process that provides a uniform composition of a barrier layer along the depth of the contact hole or via.